Field of the Invention
The invention lies in the field of materials, and relates, more specifically, to a wearing part, in particular a cutting insert, made from hard metal or cermet as the base material. The part has a hard-material coating which comprises one or more coats, a single-layer or lamellar multilayer mixed-oxide coat. The latter substantially determines the wear and comprises predominantly aluminum oxide.
The effect of hard-material coats on hard metal and on cermets of increasing the resistance to wear has been utilized at a commercial level for many years. Among the many hard-material phases which have by now been used to protect against wear, hard materials selected from the group consisting of the carbides and carbonitrides and nitrides, and also hard materials selected from the group consisting of the oxides, have proven particularly successful and are nowadays often used together, as protective layers which complement one another, in a layered coat sequence.
It is thereby customary for coats belonging to one hard-material group to be designed in lamellar form, as a plurality of individual layers alternately comprising different hard materials, in order to satisfy the various demands imposed on a wearing part, in particular also on a cutting insert for the machining of metallic workpieces, with regard to adhesion, toughness, and low wear.
There are a number of known difficulties which arise when producing usable aluminum oxide coats on hard metal cutting inserts. The coat becomes considerably more brittle as its thickness increases and as the coat deposition rate increases, on account of coat grain coarsening. The high coating temperatures which are required in order to achieve the desired aluminum oxide phase (proportion of xcex1 and/or xcexa Al2O3) and coat structure conceal the risk of permanent quality losses for the base material; moreover, they lead to increased thermal stresses in the coat.
The permanent demand for oxide coats with a wear resistance that is improved compared to that of pure Al2O3, in particular for cutting inserts, has in the past been satisfied in various ways.
For example, U.S. Pat. No. 4,052,530 (see German application DE A 27 18 647) describes the application of a mixed coating comprising Al2O3 and TiO2 phases on tools for material-removing machining. In accordance with the embodiment described in more detail in that document, the titanium oxide concentration in the coating is 2-10%, and this component is in the form of its own second phase in the aluminum oxide, as dispersed, hexagonal xcex1-titanium oxide. The increases in service life which can be achieved for cutting tools coated in this manner compared to pure Al2O3 no longer match the standard wear resistance for cutting inserts which can nowadays be achieved in other ways.
U.S. Pat. No. 4,746,563 (see European patent EP 0 162 656) describes hard metals that are provided with a multilayer coating, divided into undercoat and top coat. It is possible for each of the two coats in turn to be divided into a plurality of layers of different hard-material composition. The multilayer outer coat, with an overall layer thickness of 3 to 20 xcexcm, comprises a plurality of Al2O3 layers each with a thickness of from 0.01 to 2 xcexcm, it being possible for the Al2O3 phase to contain up to 30% by volume of titanium dioxide, the individual Al2O3 layers of the outer coat being interrupted or detached by interlayers of Ti(C, N, O, B) and SiC, AlN and AlON which are from 0.1 to 2 xcexcm thick. According to the exemplary embodiment, the Al2O3 CVD coating takes place at a furnace temperature of 1000xc2x0 C.
In view of the coat thicknesses, which are given as 3 to 20 xcexcm, these coat sequences and deposition conditions indicate long coating times and therefore a time-consuming, expensive coating process. Moreover, the long coating times lead to losses of quality in the sintered carbide base material.
U.S. Pat. No. 4,599,281 (see European patent EP 0 083 043), as a further example of the development of Al2O3 coats, relates to a wearing part, in particular a hard metal cutting insert for material-removing machining, having a multicoat hard-material coating, at least one coat comprising oxides, nitrides, carbides and/or borides using individual metals such as titanium, zirconium and hafnium which are applied alternately with at least one aluminum/boron mixed oxide coat with boron contents of from 0.01 to 1% by weight.
Wearing parts which have been coated in this manner are now in commercial use in practice. Nevertheless, there is a need for a further increase in wear resistance or service life and for production costs for coated cutting inserts to be reduced. The cutting edge stability or resistance to flaking of the cutting edges is very important for reliable, nondestructive machining of workpieces and, compared to this prior art, is considered to be urgently in need of improvement.
It is accordingly an object of the invention to provide a wearing part of hard metal with a mixed oxide coating, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which satisfies the ongoing demand for improved wear resistance and cutting edge stability for wearing parts, in particular for cutting inserts for material-removing machining, while further reducing production costs.
With the foregoing and other objects in view there is provided, in accordance with the invention, a wearing part, comprising:
a base material selected from the group consisting of hard metal and cermet;
a hard-material coating on said base material, said coating having at least one mixed-oxide layer predominantly formed of Al2O3 and substantially determining a wear of said hard-material coating, said mixed-oxide layer having a thickness of 0.5 to 10 xcexcm containing an addition of from 0.1 to  less than 3% by weight of TiO2 and 0.01 to 0.5% by weight of B2O3, and said addition is substantially dissolved and homogeneously distributed in the Al2O3 phase.
In other words, a wearing part of the generic type specified in the introduction is further improved with the single-layer or multilayer Al2O3 mixed oxide coat that is 0.5 to 10 xcexcm thick and contains, substantially dissolved and homogeneously distributed in the Al2O3 phase, an addition of 0.1 to xe2x89xa63% by weight of TiO2 and 0.01 to 0.5% by weight of B2O3.
The proportions by weight of titanium oxide and boron oxide in the Al2O3 mixed-oxide coat are based on the compounds TiO2 and B2O3. Therefore, there is no specific statement made as to the compound which is actually present.
The Al2O3 in the mixed-oxide coat may comprise one or more Al2O3 modifications, for example xcex1 Al2O3, xcexa Al2O3 or others.
The definition of the maximum additions of titanium oxide and boron oxide which can be introduced into the Al2O3 phase and the production parameters to be applied are to be adapted to one another, using rules which are known to the person skilled in the art, in such a way that the additions are actually completely dissolved in the Al2O3 or are to some extent homogeneously distributed in the Al2O3 so finely that they cannot be detected under light microscopy as an independent titanium oxide or boron oxide phase in addition to the Al2O3 phase.
The term xe2x80x9csubstantiallyxe2x80x9d which is used for the additions within the feature xe2x80x9csubstantially dissolved and homogeneously distributedxe2x80x9d therefore defines an Al2O3 mixed oxide phase within which a Ti oxide or B oxide phase cannot be detected under light microscopy, irrespective of what proportion of the addition actually satisfies the conditions of a solid solution.
A wearing part with an Al2O3 top coat formed in this manner is distinguished by relatively short production times and therefore lower production costs.
At the same time, the Al2O3 mixed oxide coat which has been modified in accordance with the invention leads to an unexpected increase in the resistance to wear, as is plausibly demonstrated below with reference to exemplary embodiments.
With coat deposition conditions which are otherwise directly comparable, with the Al2O3 mixed oxide coat according to the invention it is possible to achieve a much higher deposition rate than for Al2O3 mixed oxide coats according to the prior art. However, higher deposition rates mean a reduction in the coating time or a reduction in the coating temperature for forming a coat of defined thickness.
Higher deposition rates are however only technically appropriate if, and only to such an extent that, the grain size of a coat does not increase unacceptably as a result, since it is known that fine-grained layers tend to have a higher resistance to wear and improved toughness than coarse-grained layers.
Surprisingly, the deposition of the Al2O3 mixed oxide layers which have been modified in accordance with the invention leads to an increase in the deposition rate to an extent of up to a factor of three, without the increase in the grain size which would usually be expected by those of skill in the pertinent art.
The general rule is that the deposition rate when using CVD, PACVD (plasma activated chemical vapor deposition) and PVD coating, tends to increase as the temperature in the reaction chamber or at the surface of the wearing part increases.
The use of the above-described, surprisingly high deposition rates for the Al2O3 mixed oxide coats in accordance with the present invention makes it possible to reduce the reaction temperature in the coating chamber, for example, in the case of thermal CVD coating, from 1050xc2x0 C. to 990xc2x0 C. and below yet nevertheless to achieve a deposition rate which, although lower, remains sufficiently high with regard to achieving an acceptably short process duration as before. The reduction in each of the two parameters, both the deposition rate and the temperature in the coating chamber, however, leads to the grains in the deposited coat becoming finer to a relatively significant extent, and therefore to a highly advantageous increase in the resistance to wear with an increased cutting edge stability. Deposition at lower temperature reduces the load on the cutting material and, in addition, reduces thermal stresses in the coat.
Therefore, when advantageous use is made of the conditions described above, for the wearing part according to the invention it is possible to achieve both a significant increase in wear resistance, toughness and edge stability, on account of finer coat grain size or on account of the procedure taking place at lower temperatures, and to significantly improve the economics on account of shorter coating times.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a hard metal wearing part with mixed oxide coating, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments and examples when read in connection with the accompanying drawings.